A typical cell cycle of a eukaryotic cell includes the M phase, which includes nuclear division (mitosis) and cytoplasmic division or cytokinesis and interphase, which begins with the G1 phase, proceeds into the S phase and ends with the G2 phase, which continues until mitosis begins, initiating the next M phase. In the S phase, DNA replication and histone synthesis occurs, while in the G1 and G2 phases, no net DNA synthesis occurs, although damaged DNA can be repaired. There are several key changes which occur during the cell cycle, including a critical point in the G1 phase called the restriction point or start, beyond which a cell is committed to completing the S, G2 and M phases.
Onset of the M phase appears to be regulated by a common mechanism in all eukaryotic cells. A key element of this mechanism is the protein kinase p34.sup.cdc2, whose activation requires changes in phosphorylation and interaction with proteins referred to as cyclins, which also have an ongoing role in the M phase after activation.
Cyclins A and B were first identified in marine invertebrates as proteins that underwent periodic cycles of synthesis and degradation, with synthesis occurring during the S and G.sub.2 phases of the cell cycle followed by their abrupt degradation during mitosis (1). It is now well established that cyclin B, in association with its kinase catalytic subunit cdc2, plays a critical role in regulating the G.sub.2 /M transition in mammalian cells (2,3 and references therein). While cyclin A can substitute for cyclin B to produce an active cyclin A/cdc2 kinase, this cyclin also interacts with the related kinase cdk2 (4-6) and most probably provides a function required for the completion of DNA synthesis (7-11).
Several other mammalian cyclins designated C, D1 and E have been isolated based on their ability to complement yeast strains containing conditional G.sub.1 cyclin (CLN) genes (12-14). Although human cyclins A and B were also capable of complementing CLN function in yeast (14), cyclin E has recently been reported to associate with the cdk2 kinase during G.sub.1 of the cell cycle (11,15,16) and cyclin E/cdk2 activity appears to be necessary for entry of human fibroblasts into S phase (11). Reverse genetics was also used to identify cyclin D1 (designated PRAD 1) as a gene translocated by chromosomal inversion adjacent to the regulatory region of the parathyroid hormone gene in parathyroid adenomas (17) and in B cell leukemias and lymphomas (18). The lack of cyclin D1 mRNA expression in myeloid and lymphoid cell lines varied, but generally low levels of expression in a variety of established cell lines (18-22) suggested that inappropriate expression of this cyclin might contribute to unregulated cell growth.
Based on sequence comparisons with cyclins A, B, C and E, and the isolation of additional related cyclins D2 and D3 (originally designated CYL2 and CYL3) by low stringency hybridization with cyclin D1 probes (19,20), it now appears that the D-type cyclins represent a distinct family of proteins (12,19). Unlike other known cyclins, immune complexes isolated with cyclin D1 antibodies were reported to lack detectable histone H1 or casein kinase activity (19), suggesting that perhaps this cyclin played a role within the cell distinct from the proposed role of other classes of cyclins. In addition, we and others (23) had noted that primary diploid cells that were capable of entering fully a state of quiescence in response to growth factor deprivation, expressed higher levels of this cyclin than transformed cells.
D-type cyclins, including cyclin D1/prad1, have been identified and isolated (38,39). These were thought to serve as control elements for the start of the cell cycle by activating, in the G1 phase, a protein kinase which is essential for cell cycle start (39). Other researchers have presented evidence that cyclin D1 is involved in the G1-S phase transition in normal fibroblasts (40).
A better understanding of the elements involved in cell cycle regulation and of their interactions would contribute to a better understanding of cell proliferation. These cell cycle regulators could be used to alter or control the process of cell proliferation.